I have a three gallon tank with a filter that I've been keeping Poseidon in. He's in a quarantine bowl right now getting treated for fin rot (Salt baths and Bettafix). I was told that an uncycled tank can cause fin rot. So I figure it may be a good time to start cycling the tank.

How long does it take? What do I need to buy? What do I do exactly?
Thank you guys so much!

On tanks smaller than ten gallons, I would not attempt to (cycle ) with fish . The water volume is small and toxins will rise too quickly and fish will suffer. Once, maybe twice daily water changes would be needed and feeding of fish would need to be at minimum,perhaps once every other day. This causes no harm to fish who can go a week without food if need be. Feeding more only promotes ammonia and in smaller tanks, Fish succumb quickly.
Perhaps easiest way, is to take one small uncooked raw shrimp and in tank smaller than 10 gal.,, use approx one third of the shrimp. Be sure heater and filter are running and do not disturb ,clean,or replace the filter or water for the next three weeks. Place the piece of raw uncooked shrimp in a piece of nylon such as toe section of pantyhose with a small rock to hold it down.
Leave the shrimp in the tank for the three weeks. The bacteria will begin working as the shrimp decays . The shrimp will no doubt cause tank to smell but at the end of three weeks,you can perform a 60 percent water change and place one or two small fish in the tank on that day and then perform weekly water changes of 25 percent from this point on.
Others use a pinch of fish food every other day as opposed to using shrimp. Does the smae thing but perhaps not as smelly ,or maybe so.
Others use drops of ammonia (raw with no additives or detergents) and dose the tank daily while testing every couple days.
Which ever way you choose,it is important NOT to clean, replace.or otherwise disturb the filter during the process for that is where the majority of bacteria(good kind) will be forming.

If you know someone with a established aquarium, you may be able to talk them into sharing some of their filter material that already has a colony of bacteria(good kind) and place this in your filter and it will speed the process along. If your filter is sponge filter,or too small to allow the borrowed filter material from your friends tank to fit, then placing it in a section of nylon with a rock ,and then placing it near your filter ,or hanging it in front of outflow on your filter will work.
It is VERY, VERY, important that borrowed filter material from friends tank be kept wet in AQUARIUM water And place as quickly as possible into your tank. In this way the bacteria will remain alive . If not kept wet in AQUARIUM water , bacteria (good kind) will die off and be useless for the cycling process.
Once the tank has cycled,, then weekly 25 percent water changes and NOT over feeding will be all that is needed to provide a suitable enviornment for your fish.
Again ,during the (cycling) period,, do NOT disturb the filter material ,or clean it, or replace it. To do so will cause (cycling) to stall and tank will take much longer to mature or (cycle)
If you are fortunate enough to have a friend donate some of their filter material from their tank, then upon placing this material in YOURm tank as described,,will allow you to place your fish in the tank at that time. placing the fish in the tank is called for to help keep the bacteria(good kind) alive. It needs fish waste or fish food to remain active or it will be useless. Hope some of this helps and I believe you will find that once the heated ,(cycled),Filtered tank has matured,,you will have far less issues with sick fish as long as the weekly water changes take place on regular basis and the fish is not overfed. Good Luck.

Thank you so much for posting this and thank you so much 1077 for your reply! =] So I'm assuming that you should buy, supply, and cycle your tank before you buy the fish. correct? =] and would a submersible filter and submersible heater be good choices for a 3 gallon or would they take up too much room??